Notify & enable users to stay in the warm shell

main/command/src/main/scala/sbt/MainLoop.scala

@@ -65,6 +65,13 @@ object MainLoop {
       val newLogging = state.globalLogging.newLogger(out, logBacking)
       transferLevels(state, newLogging)
       val loggedState = state.copy(globalLogging = newLogging)
+      def isInteractive = System.console() != null
+      def hasShell = state.remainingCommands contains "shell"
+      if (isInteractive && !hasShell) {
+        state.log warn "Executing in batch mode."
+        state.log warn "  For better performance, hit [ENTER] to switch to interactive mode, or"
+        state.log warn "  consider launching sbt without any commands, or explicitly passing 'shell'"
+      }
       try run(loggedState) finally out.close()
     }
 

main/command/src/main/scala/sbt/State.scala

@@ -176,13 +176,18 @@ object State {
 
   /** Provides operations and transformations on State. */
   implicit def stateOps(s: State): StateOps = new StateOps {
-    def process(f: (String, State) => State): State =
+    def process(f: (String, State) => State): State = {
+      def doX(x: String, xs: Seq[String]) = {
+        log.debug(s"> $x")
+        f(x, s.copy(remainingCommands = xs, history = x :: s.history))
+      }
+      def isInteractive = System.console() != null
+      def hasInput = System.console().reader().ready()
       s.remainingCommands match {
-        case Seq() => exit(true)
-        case Seq(x, xs @ _*) =>
-          log.debug(s"> $x")
-          f(x, s.copy(remainingCommands = xs, history = x :: s.history))
+        case Seq()           => if (isInteractive && hasInput) doX("shell", Nil) else exit(true)
+        case Seq(x, xs @ _*) => doX(x, xs)
       }
+    }
     def :::(newCommands: Seq[String]): State = s.copy(remainingCommands = newCommands ++ s.remainingCommands)
     def ::(command: String): State = (command :: Nil) ::: this
     def ++(newCommands: Seq[Command]): State = s.copy(definedCommands = (s.definedCommands ++ newCommands).distinct)

notes/0.13.14/stay-in-shell.md

@@ -0,0 +1,7 @@
+### Improvements
+
+- Notifies & enables users to stay in sbt's shell on the warm JVM by hitting \[ENTER\] while sbt is running. [#2987][]/[#2996][] by [@dwijnand][]
+
+[#2987]: https://github.com/sbt/sbt/issues/2987
+[#2996]: https://github.com/sbt/sbt/pull/2996
+[@dwijnand]: https://github.com/dwijnand